musb: Update usb-compat to work with struct usb_device without a parent ptr

When building with CONFIG_DM_USB=y struct usb_device does not have a parent
pointer. This commit adds support to the musb code to deal with this.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Simon Glass <sjg@chromium.org>